Numerous prior designs of ossicular prostheses have sought to establish functional continuity of the ossicular chain after damage to some or all of the ossicles secondary to disease or congenital anomaly. Frequently, when the stapes is damaged or diseased, re-establishment of a connection to the inner ear via stapedectomy or stapedotomy is also required. Prostheses most often are designed to attach to remaining healthy ossicles and replace the function of the ossicle(s) which are missing or diseased.
In the case of the stapes, dysfunction often occurs as a result of a disease process called otosclerosis in which the footplate of the stapes becomes fused to the surrounding bone and no longer effectively transmits ossicular motion to the fluids of the inner ear. Most stapes prostheses over the last several decades have been constructed of either a circumferential or nearly circumferential wire loop or a broader ribbon-type (flat wire) loop which directly attaches to the long process of the incus. More recently, clip-type prostheses have been developed which are comprised of a complex-shape broad ribbon in which a more limited surface area of the incus is placed in contact with the flattened clip. In either case, the component for incus attachment is joined to a piston which fits into a hole (fenestra) in the stapes footplate created by the surgeon so that the piston can translate ossicular movement to the fluids of the inner ear.
Ossicular prostheses which have been designed to replace the function of the stapes and attach to the incus have frequently caused necrosis/erosion (loss of bone tissue) in the area of the incus to which they are attached, as well as the bone more distal to the prosthesis attachment. This bone loss in the area of the prosthesis attachment can cause loosening of the prosthesis and resultant hearing loss because of piston migration out of the hole in the stapes footplate and/or de-coupling of the motion from the incus to the prosthesis. Ideally, the ossicular prosthesis which is designed to replace the function of the stapes will attach firmly and permanently to the long process of the incus without ever loosening or migrating away from its position of placement at the time of surgery. This is also true of prostheses which attach to the malleus and replace the function of the other ossicles.
Bone/tissue loss in the area of the attachment of the stapes prosthesis to the incus is thought to be due to impaired blood supply to the long process of the incus after surgery. Prior to surgery, blood is supplied to the area of the long process from both the body of the incus distally to the long process, as well as from the opposite direction across the joint where the incus is joined to the stapes. Since the attachment between the stapes and incus is cut during surgery, the only blood supply to the long process of the incus after surgery flows distally to the long process from the body of the incus. Accordingly, blood flow to supply the area of the incus both at and distal to the prosthesis attachment ideally should not be restricted by the prosthesis itself. It is widely believed that a prosthesis which exerts sufficient mechanical pressure on blood vessels/mucosa near the surface of the incus can cause a restriction of blood flow sufficient to cause incus necrosis and subsequent failure of the prosthesis.
Since the blood supply in the area of the prosthesis attachment is tenuous after surgery and most prostheses have areas of very broad or circumferential contact points with the long process, relatively large areas where blood vessels or delicate mucosa are located can be compressed by the prosthesis and cause death of the underlying or distal tissues. Efforts to reduce both the amount and the surface area of mechanical pressure exerted on the incus have resulted in new prostheses with broader and/or softer ribbons to contact the incus; new methods of crimping to achieve optimal pressure on the incus (tight enough to ensure good coupling of incus translation to that of the piston in the inner ear, but loose enough to reduce the chance of pressure necrosis); or a change in the shape of the ribbon or wire so that only opposing sides of the incus are grasped firmly. Nevertheless, all efforts to date share the common characteristic that at least some of the wire or ribbon is broadly in contact with the incus and consequently have the potential to cause pressure necrosis to the underlying mucosa and loss of bone in these areas of broad contact.
In the case of prostheses which attach to the malleus to replace the function of the incus, stapes, or both, the prosthesis most often is placed in contact with the medial portion of the malleus. Contact is not usually made with the lateral aspect of the malleus because its lateral surface is integrated with the ear drum. The prosthesis component which contacts the malleus usually comprises a flat disc with a shallow depression in which the medial malleus lies. Commonly, the prosthesis “falls over” after surgery with a loss of contact with the malleus, with resultant hearing loss. Continuing malleus-prosthesis contact after surgery is initially dependent solely on the medial pressure the malleus exerts on the prosthesis.